1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus having a shiftable developing means.
2. Related Background Art
In the past, a color image forming apparatus in which a plurality of different color toner images are successively transferred onto a single transfer material in a superimposed fashion by using a plurality of developing devices containing different color toners and the color images are fixed to the transfer material to obtain a full-color image has widely been used. FIG. 6 shows a conventional color image forming apparatus. Now, such a color image forming apparatus will be described.
FIG. 6 is a view showing an entire construction of a color laser printer as an example of a color image forming apparatus.
An image forming portion of the color laser printer is constituted by a photosensitive drum (image bearing member) 45 rotated at a constant speed, a fixed black developing device 49B, and three rotatable color developing devices (yellow developing device 48Y, magenta developing device 48M and cyan developing device 48C).
Below the image forming portion, there is disposed an intermediate transfer member 43 for holding the developed color toner images in a superimposed fashion and for transferring the color toner images onto a transfer material 42 fed from a feeding portion.
The transfer material 42 to which the color images were transferred is sent to a fixing portion 52, where the color images are fixed to the transfer material 42. Thereafter, the transfer material is discharged onto a discharge portion 61 provided on an upper surface of the printer via pairs of discharge rollers 58, 59, 60.
The photosensitive drum 45 is constituted by coating an organic photo-conductor layer on an outer surface of an aluminium cylinder having a diameter of about 62 mm, and a cleaner blade 46 and a primary charging means 47 are disposed around the photosensitive drum.
The photosensitive drum 45 is rotated in a counter clockwise direction in FIG. 6 in synchronous with an image forming operation by transmitting a driving force of a drive motor (not shown) provided at a rear side in FIG. 6 to the photosensitive drum.
Exposure to the photosensitive drum 45 is effected by a scanner portion 57. That is to say, when an image signal is sent to a laser diode, the laser diode illuminates light onto a polygon mirror at a timing corresponding to the image signal. The polygon mirror is rotated at a high speed by a scanner motor so that the light reflected from the polygon mirror can selectively expose the surface of the photosensitive drum 45 through a focusing lens and a reflection mirror, thereby forming an electrostatic image on the photosensitive drum 45.
The electrostatic image is visualized by three rotatable developing devices 48Y, 48M, 48C and a black developing device 49B which perform yellow color development, magenta color development, cyan color development and black color development.
The black developing device 49B is fixed at a developing position, except that this developing device is mounted to a main body of the printer. The black developing device serves to form a visualized image with black toner.
A toner containing capacity of the black developing device 49B is selected to be greater than toner containing capacities of the other developing devices 48Y, 48M, 48C by about two times or more to permit printing of 15,000 sheets (A4 size sheet, printing percentage=5%) in consideration of kinds of documents handled by an operator, image patterns and the amount of toner consumed.
By increasing the toner containing capacity of the black developing device in this way, frequency for exchanging the black developing device is reduced, and running cost for each print can also be reduced.
As shown in FIG. 6, the black developing device is disposed between the laser scanner and the three rotatable developing devices 48Y, 48M, 48C. With this arrangement, the toners leaking when the developing devices 48Y, 48M, 48C are rotated are prevented from scattering onto the optical parts such as the laser scanner. In this manner, the toner is prevented from adhering to the polygon mirror, lens and mirror to worsen electrostatic image formation, thereby obtaining a sharp output image.
Each of the three rotatable developing devices 48Y, 48M, 48C contains the toner for permitting printing of 6,000 sheets (A4 size sheet, printing percentage=5%). The three rotatable developing devices are held detachably attachable to a developing rotary 51 rotated around a shaft 50.
In the image formation, the developing devices are rotatingly shifted around the shaft 50 in the condition that they are held on the developing rotary 51 so that the pre-selected developing device is stopped at a position opposed to the photosensitive drum 45. In this case, the visualized image corresponding to the electrostatic image on the photosensitive drum 45 is formed.
In the color image formation, whenever the intermediate transfer member 43 is rotated by one revolution, the developing rotary 51 is rotated so that developing processes of the yellow developing device 48Y, magenta developing device 48M, cyan developing device 48C and black developing device 49B are successively effected in order.
For example, when the rotatable yellow developing device 48Y is positioned and stopped at the position opposed to the photosensitive drum 45, the rotatable yellow developing device 48Y effects development on the photosensitive drum 45 with yellow toner to form an yellow toner image. Similarly, regarding the magenta developing device 48M and the cyan developing device 48C, the color toner development is effected in the same mechanism. Application of bias and transmission of driving force to developing rollers of the rotatable developing devices 48Y, 48M, 48C are performed when each of the developing devices is rotatingly shifted to the developing position.
The intermediate transfer member 43 serves to receive the toner images on the photosensitive drum 45 visualized by the developing devices during the color image formation by four times (yellow, magenta, cyan and black color images) in a superimposed fashion. The intermediate transfer member 43 is rotated in a clockwise direction in FIG. 6 in synchronous with the outer peripheral speed of the photosensitive drum 45.
The intermediate transfer member 43 to which the toner images were transferred in the superimposed fashion cooperates with a transfer roller 44 (to which voltage is applied) to pinch and convey the transfer material 42 therebetween, with the result that the color toner images on the intermediate transfer member 42 are collectively transferred onto the transfer material 42.
In this example, the intermediate transfer member 43 is constituted by coating an elastic layer made of middle resistance sponge or middle resistance rubber on an aluminium cylinder having a diameter of 186 mm.
A cleaning means serves to remove residual toner remaining on the photosensitive drum 45 after the toner image on the photosensitive drum 45 visualized by the developing means were transferred to the intermediate transfer member 43.
While the four color toner images are being formed on the intermediate transfer member 43, i.e., while the intermediate transfer member 43 is being rotated by several revolutions, as shown by the solid line in FIG. 6, the transfer roller 44 is located at a lower position so that the roller is spaced apart from the intermediate transfer member 43 not to distort the images.
After the four color toner images were formed on the intermediate transfer member 43, as shown by the two dots and chain line in FIG. 6, the transfer roller 44 is shifted to an upper position where the roller is urged against the intermediate transfer member 43 with predetermined pressure, at a timing corresponding to the transferring of the color images onto the transfer material 42. At the same time, bias is applied to the transfer roller 44, thereby transferring the toner images on the intermediate transfer member 43 onto the transfer material 42.
The transfer material 42 pinched between the intermediate transfer member 43 and the transfer roller 44 is subjected to the transferring process, and, at the same time, the transfer material is conveyed to the left in FIG. 6 at a predetermined speed toward the fixing device.
The fixing portion 52 serves to fix the toner images formed in the previous process. As shown in FIG. 6, the fixing portion 52 comprises a fixing roller 53 for applying heat to the transfer material 42, and a pressure roller 54 for urging the transfer material 42 against the fixing roller 53. The rollers are hollow rollers having heaters 55, 56 therein. By rotating the rollers, the transfer material 42 is conveyed. That is to say, the transfer material 42 bearing the toner images is conveyed by the fixing roller 53 and the pressure roller 54; meanwhile, by applying heat and pressure to the transfer material, the toner is fixed to the transfer material 42.
The toner images are melted and mixed by the fixing device 52 to form a full-color image fixed to the transfer material. Thereafter, the transfer material is discharged out of the printer.
The image forming apparatus having the above-mentioned construction has the following problems. Briefly explaining, since the charging device 47, image signal supplying position, rotary developing apparatus 50 and cleaner 46 which are disposed around the photosensitive drum 45 cannot be changed in their order or sequence, if the apparatus tries to be made more compact, the developing apparatus will be arranged in the vicinity of the fixing apparatus. Particularly, in an apparatus using a transfer drum (intermediate transfer member), the rotary developing apparatus may be disposed above the fixing apparatus.
There has widely been used a fixing apparatus comprising a fixing roller in which a surface temperature thereof is maintained to about 140.degree. C. to 180.degree. C. and a pressure roller urged against the fixing roller and wherein a transfer material bearing non-fixed toner images is passed through a fixing nip portion between these rollers. However, when such a fixing apparatus is used, a temperature of an area above the fixing apparatus is naturally increased, with the result that the toner contained within the rotary developing apparatus disposed in such an area (particularly, developing device positioned in the vicinity of the fixing apparatus) may be softened. Such a condition may easily occur when the rotary developing apparatus is stopped with particular developing device facing to the fixing apparatus and the fixing apparatus is in an operative condition or a stand-by condition to control temperatures of the fixing and pressure rollers to predetermined temperatures.
Normally, the toner is formed as powder including particles having diameters of about several .mu.m to some dozen .mu.m obtained by crushing or suspension-porimerizing resin (such as polyester or acrylic styrene) dispersing color pigment therein and is generally softened at a temperature of about 60.degree. C. and is completely melted at a temperature of 90.degree. C. to 100.degree. C.
If the toner in the developing device is softened, the softened toner will be aggregated or solidified within the developing device later to make the image formation unstable, thereby worsening the image quality.